11 Answers
11

Daggers are indeed faster, although I can't find a reference on how much faster. Everything (enchantments, perks, etc.) considered, I don't think there is a huge difference, so it should come down more to style.

and both haveArmsman: +20/40/60/80/100% damage with a one-handed weapon

So there's a difference here, in that the perks favor different playing styles: daggers benefit from stealth and swords benefit from more all-purpose combat. If you're dual-wielding a dagger and sword, that shouldn't really matter as you can sneak attack with the dagger and follow up with both.

Speed
As you get better weapons, the damage difference as a percentage falls. For example, an Iron Sword deals 7 base damage vs an Iron Dagger's 4, which is 75% stronger. A Daedric Sword deals 14 vs the Dagger's 11, so only 27% more. Smithed to legendary, it's 24 versus 21, so only 14% difference. I don't know how much the crits from Bladesman add.

Also, because daggers are faster, they will be able to deal out on-hit damage from enchantments more quickly. If you're focusing at all on enchanting, you may want to consider that.

An actual sneak attack
In my experience, a standing dual-wielded sneak power attack with two daggers will connect and get the sneak bonus on the first two and maybe more (most enemies are dead by then). I can't tell whether that's up to 60x normal damage (15x from Assassin's Blade, 2x from Shrouded Gloves, 2x from power attack) or if there's some cap, but suffice it to say that if you planning to be very sneaky, not much will hold up to such an attack with two Daedric Daggers.

On the other hand, daggers do not benefit from several bonuses to swords. That includes Fortify One-Handed, Bladesman, etc. That includes silly things like the Nightingale Gloves.

Bottom line
If you don't plan to be seen often, I personally have stuck with daggers. My strategy for dealing with non-sneaking situations is falling back on magic. If you want more damage in face-to-face combat, of course, consider a sword, just keep in mind that the ideal equipment for sneaking is quite different than that for combat.

@yx Really. They benefit from Armsman but not any alchemical/enchanted Fortify One-Handed. It may be an oversight that will be fixed or it may be intentional. Here's one discussion on it. For something more authoritative, here's UESP.
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brymckDec 1 '11 at 12:11

Not a big enough advantage to justify it on it's own, but I find a forward power attack with a mainhand dagger to cover more ground than any of the other 'charge' options.
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LessPop_MoreFizz♦Dec 5 '11 at 20:55

Daggers are much lighter than swords. If you plan on carrying a wide variety of enchantments you can carry more daggers with different enchantmrnts than swords without over encumbering yourself.

I play a mage that uses enchanted daggers as utility weapons. Mainly only as tools for applying magic effects through enchantments and poisons. Not for the actual physical damage. They are the best weapon for the job.

It is nice because a lot of magic is only available as enchantments like the absorb and damage attribute enchantments. They also work as a good back up for if your magika runs low.

One combo ive used in past is blade of woe and mehrunes dagger as it combined the strength of the blade of woe, the quick flurry of attacks avaliable from daggers, and the one hit kill chance of mehrunes dagger. Used with cicero 's gloves when sneaking it allows over 1k damage.

I know it sounds weird but even though you waste like 1 second on it, the dagger power attack does look cool. Also when sneaking daggers are the only silent weapon.

My current loadout is candle light in left and ebony dagger in right and I can clear out a cave full of forsworn in a couple of minutes. If I'm in a tricky situation like a Draugr Death Lord, then I may equip my Blades sword. Overall I think daggers are fine, but the dagger sword combo is also worth looking into.

You may have two daggers of woe but you still only do 15x damage not 30x when you sneak attack, the duel wield dagger animation does not allow for those results, I get 15x plus 6x with dagger sword combo & it kills a Deathlord outright on Master hardness. Add invisibility to it & you can sneak up on anything. sorry, it just doesn't matter that you have two of them (Blades of Woe). Chillrend & woe are best combo IMO; standing dual power attack with handwraps & all related perks will give me 12x + 30x for instant kill anything (so far) except maybe named Dragon Priest & dragons which I haven't been able to sneak attack yet. Only thing better would to be an Orc & pot the Berserk!

Plus Bladesman is unnecessary, the crit damage is negligible at best for sneak damage but it is prob good when direct attacking with duel swords. Most things die in on power attack from front with my dagger sword/combo.
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KalidenzaApr 29 '12 at 7:48

Swords do the highest base DPS of all the hand weapons. Daggers pretty much Only work well for Assassin's Blade, and speeding up dual attacks. Also, the offhand is slightly slower than the right with a comparable weapon, so a Sword/Dagger combo will have the same speed in the appropriate hands. Also works with Axe/Sword, or Mace/Ace, btw. Don't forget Reach, either. Swords tend to have the longest, while daggers are the shortest in 1H.

Well, if you can steal or pickpocket equipped weapons take the blade of woe from Astrid of the Darkbrotherhood, then complete then questline and she'll give it to you.t With two blades of woe (blades of woe are considerably stronger than daedric daggers) at 15 times dames the dual wield power attack will reduce even a Draugr Deathlord to approximately 30% health then use the shout ice form to freeze it wile its not facing you could easily keep you partially hidden. Then just roll back into a corner before he defrosts. Then repeat works on anything smaller than a mammoth or giant.

Bryan is correct. Attack speed of power attacks when dual wielding is decided by left hand weapon. That being said, maximize the fastest attack speed when dual wielding you actually have to have a sword in left hand and a dagger in your right, not 2 daggers which would make more sense.

The reason for this is that the animation for power attacks with 2 daggers is badly made so it takes about 1 second before you actually start attacking with 2 daggers because your character rotates the daggers around before attacking.

On another note, having a dagger in your left hand and a mace in your right hand power attacks would be determined by the dagger. However, if you plan to slit throats (animation)
keep in mind that can ONLY happen if you have a dagger or a sword in your right hand. It's not possible with maces or axes.

Lastly, not many people seem to know this but regardless of what weapon you have in your right hand: dagger, sword, axe, mace, swinging with your right hand while dual wielding if you have dual flurry perks makes you swing the right hand weapon faster as well (not power attacks or hitting both buttons at once) only single weapon swings with right hand.

It appears that the attack speed of the dual-strike attack (activated by hitting both attack buttons at once) is based on your off-hand (left hand, right click with mouse). This means that, if you equip a dagger in your offhand, the optimal dual wield weapon to pair it with in your mainhand is a mace. This allows you to take advantage of the maces higher base damage, while attacking faster than would normally be possible.

In combat, daggers are useful against blocking enemies, since (in my experience) attacking a blocking enemy with a dagger will result in a quick 'punch' over top of their shield/weapon, causing them to drop their block and leaving them open for power attacks. All other weapons require a power attack to break a block. So it isn't entirely useless in combat, so long as you have a different 2nd weapon.